Abstract: A cylinder tumbler lock mechanism comprises a plug having a row of slots formed therein, each slot containing a tumbler. The tumbler includes a formation for engagement with a side of a key. The tumbler is slidably mounted in the slot and movable in a first direction from: (i) a locking position in which one end of the tumbler projects out of an open end of the slot, into (ii) a decoding position in which further movement of the tumbler in the first direction is blocked. For at least two tumblers that differ from one another, the height of the formation, when in the decoding position, is substantially identical. Thus, the lock is made considerably more difficult to decode.